1. Field of the Invention
The present invention relates to an optimal device for measuring physiological state which is used to measure conditions in the human body. More specifically, the present invention relates to a device for analyzing pulse waves used to diagnose the circulatory system in the human body, and to a sphygmomanometer which evaluates compliance and resistance in the blood vessels at the center and periphery of the circulatory system based on the physiological state measured at the periphery of the human body, and estimates blood pressure at the center of the circulatory system.
2. Background Art
Blood pressure and heart rate are most commonly used when diagnosing the condition of the circulatory system in the human body. However, in order to carry out a more detailed diagnosis, it becomes necessary to measure the so-called circulatory state parameters of compliance and viscous resistance in the blood vessels. Moreover, in the case where these parameters are expressed using a model, a lumped four parameter model may be employed as a model for expressing the behavior of the arterial system.
The pressure waveform and blood flow volume at the proximal portion of the aorta and at the site of insertion of a catheter into an artery need to be measured in order to measure these circulatory state parameters, however. For this purpose, a direct method of measurement, in which a catheter is inserted into an artery, or an indirect method employing supersonic waves or the like, may be applied. However, the former method is invasive, and employs a large device, while the latter method, although permitting non-invasive observation of blood flow within the blood vessels, requires training and, moreover, necessitates a large device to carry out the measurements.
Accordingly, the present inventors discovered a method for approximating the parameters in a lumped four parameter model by measuring just the pulse waveform at the radius artery and the stroke volume. Thereafter, the present inventors proposed a pulsewave analysis device capable of carrying out an evaluation of the circulatory state parameters in an easy and non-invasive method by employing this method (see Japanese Patent Laid-open Publication No. Hei 6-205747, Title: Device for Analyzing Pulsewaves).
However, the aforementioned method does not employ a model which treats blood vessel compliance at the periphery and center of the arterial system separately. Accordingly, when exercising, or in cases where a pharmacological agent effecting circulatory state has been administered to a patient, it is not possible to evaluate the separate effects of that medication at the periphery and center of the arterial system.
A brief explanation will now be made of the aforementioned measurement of blood pressure.
In the non-invasive sphygmomanometer conventionally employed, a cuff is attached to the upper arm, for example, of a test subject, pressure is applied to the cuff and the pulsewave of the test subject is detected to provide a measurement of blood pressure. Japanese Patent Application Laid Open No. Hei 4-276234, for example, discloses a sphygmomanometer at the periphery of a test subject's body. Namely, as shown in FIG. 29, cuff 110 is wrapped around the upper arm of a test subject, and a band 138 is wrapped around the subject's wrist 140. Pulsewave sensor 134 is attached to the radius artery of the test subject, and the test subject's pulsewave is detected. After applying pressure to cuff 110, the conventional oscillometric method is employed to measure the systolic and diastolic pressure values as the pressure falls.
However, if blood pressure values at the periphery and center of the arterial system in the human body are actually measured, a difference in center and peripheral blood pressure values is observed, particularly in the case of the systolic pressure value. Moreover, the degree of this difference varies depending on the shape of the pulsewave which is observed at the periphery of the arterial system. FIGS. 22 through 24 are provided to explain this variation in blood pressure values according to pulsewave shape. The pressure waveform and systolic/diastolic pressure values at the aorta, which is at the center of the arterial system, and the pressure waveform and systolic/diastolic pressure values at the radius artery, which is at the periphery of the arterial system, are shown in these figures.
FIG. 22 shows the first type of pulse waveform, wherein the systolic pressure value obtained at the aorta is indicated by the dashed line and the systolic pressure value obtained from the radius artery is indicated by the solid line. Although the systolic pressure value obtained at the radius artery is slightly higher, these blood pressure values may be viewed as almost equivalent. In the case of the second type of pulse waveform shown in FIG. 23, however, the difference between the systolic pressure values obtained at the aorta and at the radius artery is 14.9 mmHg, a considerably greater difference than observed in the case of the Type I pulse waveform shown in FIG. 22. Further, in the case of the third type of pulse waveform shown in FIG. 24, the difference between the systolic pressure values is even greater, at 26.1 mmHg. Moreover, in contrast to the Type I and Type II pulse waveforms, in the case of a Type III pulse waveform, the pressure waveform obtained at the aorta is higher in its entirety than that of the pressure waveform obtained at the radius artery. Thus, based on these figures, the diastolic pressure value at the radius artery does not depend on the shape of the pulsewave, but is approximately the same for each pulsewave type.
A brief explanation will now be made of the Type I, Type II and Type III pulsewaves described above. A Type I pulse waveform is observed in a person of normal health. The waveform is relaxed and loose, and is characterized by a fixed rhythm with little disruption. On the other hand, a Type II pulse waveform demonstrates a sharp rise followed immediately by a fall. The aortic notch is deep, while the subsequent peaks in the expansion phase are significantly higher than usual. A Type III pulse waveform rises sharply, with blood pressure remaining elevated for a fixed period of time thereafter, rather than immediately falling off.
As may be gathered from these figures, it is possible for the peripheral blood pressure value obtained at the radius artery or upper arm to be elevated, while the blood pressure value obtained at the proximal portion of the aorta, i.e., at the center of the arterial system, is low. Further, the opposite situation is also possible, namely, the blood pressure value at the periphery is low, while the blood pressure value at the center of the arterial system is high. This relationship will differ depending on the shape of the pulse waveform, and, moreover, is realistically expressed in the shape of the pulse waveform.
For example, when a hypertensive agent is administered to a patient as a treatment for high blood pressure, the drug's effect is observed based on the blood pressure at the radius artery. In this case, however, it is possible that the blood pressure at the center of the arterial system is not actually reduced, even if there is a drop in the blood pressure value measured at the periphery. Accordingly, it can be difficult to correctly ascertain the drug's effect based only on the peripheral blood pressure value. Conversely, even if no change is observed in the blood pressure at the periphery of the arterial system, the actual load on the heart may in fact have been reduced if there is a change in the pressure waveform at the aorta, and the blood pressure at the center of the arterial system drops. In this case, the drug's effect has been fully expressed, even though there was no reduction in blood pressure at the periphery of the arterial system. Accordingly, it is difficult to determine this fact based only on the blood pressure at the periphery of the arterial system.
Accordingly, when measuring blood pressure, the correct approach is to observe the extent of the actual load on the heart. This is because when a determination is made based on blood pressure values measured at the periphery of the arterial system, as has been the conventional practice, there is a chance that the load on the heart will be over or under evaluated.